The present invention relates generally to an optical system comprising a diffractive surface having a lens action based on a diffraction phenomenon, and more particularly to a compact yet high-zoom-ratio zoom lens system comprising a diffractive surface, which is used with lens shutter cameras, etc.
For lens shutter cameras (hereinafter called LS cameras for short) with built-in zoom lenses, many makers have recently developed a variety of products now widely accepted to many users because of their convenience and ease of use. Thus, it appears that LS cameras have steadily spread as commodities. A variety of LS cameras having an ever higher zoom ratio are now proposed with new values added thereto. However, the body size of currently available cameras become large with increasing zoom ratios. This is not always preferable for users. To solve such size problems, on the other hand, ultra-compact cameras are put forward with much importance given to portability. However, all available cameras have both merits and demerits; compact yet high-zoom-ratio cameras are greatly needed. Thus, phototaking lens systems to meet such demands are expected.
JP-A 4-37810 discloses a three-group zoom lens system having a zoom ratio of 2.6, which comprises, in order from the object side thereof, a positive lens group, a positive lens group and a negative lens group, about 10 lenses in all. The diameter of the first lens group is reduced by locating a stop on the side of the second lens group nearest to an object, and aberrations are well corrected by the optimization of the design of the second lens group. The total length of the zoom lens system set forth therein is much shorter than ever before. However, the total length of the lens arrangement (the length from the first surface to the final surface) is still great due to many lenses.
JP-A 8-286110 discloses a three-group zoom lens system which comprises, in order from the object side thereof, a positive lens group, a positive lens group and a negative lens group, and only two lenses for each group, six lenses in all. The zoom ratio achieved is in the range of 3.0 to as high as about 3.3. However, correction of aberrations is insufficient at each lens group, and chromatic aberrations at the telephoto end in particular become too large for correction.
JP-A 5-173069 discloses a similar zoom lens system, and one example given therein is directed to a specific zoom lens system consisting of six lenses in all and having a zoom ratio as high as 3.6. The length of the lens arrangement is short due to a reduced number of lenses. However, this system is not preferable because a vitreous material showing anomalous dispersion is used for correction of aberrations, resulting in cost increases.
On the other hand, JP-A 5-188296 discloses a similar three-group zoom lens system which comprises a positive lens group, a positive lens group and a negative lens group, and achieves a zoom ratio of 2 with a lens arrangement consisting of five lenses. The length of the lens arrangement is very short with well-corrected aberrations. However, the zoom ratio is very low. These prior systems are all designed to achieve correction of aberrations by use of an aspherical glass lens, and are unavoidably expensive.
The applicant, too, has filed Japanese Patent Application No. 8-326457 to propose a three-group zoom lens system of the positive, positive and negative type, which achieves a zoom ratio of 2 with a lens arrangement consisting of five lenses. The cost of this lens system is brought down by making use of an aspherical plastic surface. Aberrations are well corrected with a reduction in the length of the lens arrangement. However, the zoom ratio is still low.
An object of the present invention is to make proper use of a diffractive surface, thereby making high the zoom ratio of a zoom lens system which is constructed of a short lens arrangement comprising a small number of lenses at low costs yet with well-corrected aberrations.
For a better understanding of the present invention, the lens action of the diffractive surface is then explained. While a conventional lens is based on the refraction of light at a medium interface, the lens action of the diffractive surface is based on the diffraction of light. Now consider the incidence of light on such a diffraction grating as shown generally in FIG. 1. Emergent light upon diffraction satisfies the following equation (a): EQU sin.theta.-sin.theta.'=m.lambda./d (a)
where .theta. is the angle of incidence, .theta.' is the exit angle, .lambda. is the wavelength of light, d is the pitch of the diffraction grating, and m is the order of diffraction. Consequently, if the pitch of a ring form of diffraction grating is properly determined according to equation (a), it is then possible to converge the incident light on one point, i.e., impart lens action to the diffraction grating. Here let r.sub.j and f the radius of a j-th ring on the grating and the focal length of the diffractive surface, respectively. Then, the following equation (b) is satisfied in a region of first approximation: EQU r.sub.j.sup.2 =2j.lambda.f (b)
For a diffraction grating, on the other hand, a bright-and-dark ring form of amplitude-modulated type grating, and a phase-modulated type grating with a variable refractive index or optical path length are known. In the amplitude-modulated type, for instance, the diffraction efficiency (defined by the ratio between the quantity of incident light and the quantity of the first order of diffracted light) is about 6% at most because plural orders of diffracted light are produced. In the phase-modulated type, too, the diffraction efficiency is about 34% at most. If the diffraction grating is modified such that its section is of such saw-toothed shape as depicted in FIG. 2, however, the diffraction efficiency can theoretically be increased to 100%. Even though actual losses are taken into account, a diffraction efficiency of at least 95% is then obtainable. Such a diffraction grating is called a kinoform. In this case, the height of each tooth is given by EQU h=m.lambda./(n-1) (c)
where h is the height of the tooth, and n is the index of refraction of a substrate.
As can be predicted from equation (c), a diffraction efficiency of 100% is achievable at only one wavelength. FIG. 3 illustrates a specific wavelength vs. diffraction efficiency relation at 550 nm design wavelength. As the wavelength goes away from the design wavelength, the diffraction efficiency decreases greatly. With decreasing diffraction efficiency, the rest of light exists as unnecessary light. In the case of an optical system used under white light, care should be taken of a flare problem due to such unnecessary light.
How to design the diffractive surface is now explained. The diffractive surface may be designed by some known methods. However, the present invention makes use of an ultra-high index method. According to this method, the diffractive surface is known to be equivalent to a refractive surface having an ultra-high refractive index at null thickness. At this time, the index of refraction n(.lambda.) at any wavelength is given by EQU n(.lambda.)=1+{n(.lambda..sub.0)-1}.lambda./.lambda..sub.0 (d)
where .lambda. is an arbitrary wavelength, .lambda..sub.0 is a reference wavelength, and n(.lambda..sub.0) is the index of refraction at wavelength .lambda..
The diffractive surface has two important features when used in the form of a lens. The first feature is aspheric action, as already noted. If the pitch of a diffraction grating is properly determined, it is then possible to converge light on one point. The second feature is that dispersion is very large or, in another parlance, a so-called Abbe's number is found to be -3.45 from equation (d). Chromatic aberrations several tens times as large as that of a conventional glass material are produced with a minus sign or in the opposite direction. It is also found that strong anomalous dispersion is obtained with a low partial dispersion ratio.
An example of applying such a diffractive surface to optical systems used under natural light is known from an article "Hybrid diffractive-lenses and achromats", Appl. Opt. 27, pp. 2960-2971. This prior publication shows an example of calculation in the case where, based on the principle of correction of paraxial chromatic aberration, the diffractive surface is used in combination with a single glass lens to make correction for longitudinal chromatic aberration. Specifically, the publication shows that the plane side of a plano-convex lens is constructed of a diffractive surface for the achievement of achromatization, and refers to the resulting secondary spectra. The publication also shows the results of achromatization by a diffractive surface and doublet combination.
U.S. Pat. No. 5,543,966 shows an example of achromatization by use of a singlet and diffractive surface combination. This example is applied to a so-called film camera for the purpose of making high the performance of a phototaking optical system comprising a positive meniscus lens convex on the subject side and a stop by disposing a diffractive surface on the image-side surface of the lens, thereby making correction for chromatic aberrations.
"Diffractive optics at Eastman Kodak Company", SPIE, Vol. 2689, pp. 227-254 shows applications of the diffractive surface to a variety of optical systems. In particular, this publication exemplifies an application of the diffractive surface of a phototaking zoom lens system for LS cameras, wherein a doublet in the first lens group in a three-group zoom lens system of the positive, positive and negative type is replaced by one diffractive element. For lack of design data, however, details of that application cannot be understood.